David Friehling, 49, entered the plea in U.S. District Court in Manhattan, who lost billions of dollars while he audited Madoff's financial records between 1991 and 2008. The plea was part of a cooperation deal with prosecutors.

Before he explained his crimes to the judge, he said he wanted to make clear: "At no time was I ever aware Bernard Madoff was engaged in a Ponzi scheme."

CBS News investigative producer Pat Milton watched the hearing Tuesday in federal court for the Southern District of New York.

The courtroom was packed with reporters and some of Madoff's victims, Milton reported. As part of Friehling's statement supporting his guilty plea, he apologized to Madoff's investors for his role in the fraud. Hellerstein provided an opportunity for investors to speak at the proceeding but no one asked to do so.

Friehling didn't face the victims when he apologized to them in open court, choosing instead to direct his statements to judge, Milton reported. He wore a black suit and a stern look on his face when he stood to tell the court that he wished to plead guilty.

He said if he had known that investor money was not properly invested, he would not have poured his family's savings, including the college funds for three children ages 17, 20 and 24, into Madoff's investment business.

But he admitted that he took the financial records handed him by Madoff "at face value," failing to independently verify the assets of Madoff's investment company or ensure that his bank account records or charts listing the purchase of securities were accurate.

He said he also prepared personal tax returns for Madoff that he knew were not accurate.

Friehling's statement was made to support his guilty plea to charges of securities fraud, investment adviser fraud, making false filings with the Securities and Exchange Commission, and obstructing or impeding the administration of the Internal Revenue laws.

The charges carry a potential prison term of up to 114 years in prison, though substantial cooperation with prosecutors can result in significant leniency.

A tentative sentencing date was set for Feb. 26, but it was unlikely that Friehling will be sentenced until he completes his cooperation with prosecutors, which will include divulging any crimes by others he knows about and testifying before grand juries investigating the fraud.

Friehling remains free on $2.5 million bail. His lawyer, Andrew Lankler, declined to comment after the plea.

Friehling was Madoff's auditor from 1991 to 2008, a job he inherited from his father-in-law, who was originally hired by the father of Madoff's wife Ruth in 1963, shortly after Madoff began his investment career.

The 71-year-old Madoff, who pleaded guilty to fraud charges in March, is serving a 150-year sentence at a prison in North Carolina.

Despite the more than $65 billion in private investments that Madoff claimed he oversaw for thousands of investors, Friehling said he operated alone out of a tiny office in suburban New City, N.Y.

Authorities say if Friehling had done his job, they would have known years earlier that Madoff was carrying out history's greatest Ponzi scheme by paying money to some investors with the proceeds he received from other investors.

When Madoff revealed his fraud last December in a confession to his sons and later to the FBI, it was discovered that only a few hundred million dollars was left of the more than $170 billion that prosecutors say went through his accounts over the years. Prosecutors say investors originally entrusted Madoff with more than $13 billion and he greatly exaggerated bogus gains.

It's the third plea in the case. Frank DiPascali, Madoff's former finance chief, also is cooperating after pleading guilty in August to helping Madoff carry out his fraud.